1. Field of the Invention
The present invention relates to implantable medical devices which produce electrical pulses that to stimulate organs of an animal, and more particularly to the storage of energy in such medical devices.
2. Description of the Related Art
A remedy for people with slowed or disrupted natural heart activity involves implanting a cardiac pacing device which is a small electronic apparatus that stimulates the heart to beat at regular rates.
Typically the pacing device is implanted in the patient's chest and has sensor electrodes that detect electrical impulses associated with in the heart contractions. These sensed impulses are analyzed to determine when abnormal cardiac activity occurs, in which event a pulse generator is triggered to produce electrical pulses. Wires carry these pulses to electrodes placed adjacent specific cardiac muscles, which when electrically stimulated contract the heart chambers. It is important that the stimulation electrodes be properly located to produce contraction of the heart chambers.
Modern cardiac pacing devices vary the stimulation to adapt the heart rate to the patient's level of activity, thereby mimicking the heart's natural action. The pulse generator modifies that rate by tracking the activity of the sinus node of the heart or by responding to other sensor signals that indicate body motion or respiration rate.
U.S. Pat. No. 6,445,953 describes a cardiac pacemaker that has a pacing device, which can be located outside the patient, to detect abnormal electrical cardiac activity. In that event, the pacing device emits a radio frequency signal, that is received by a stimulator implanted in a vein or artery of the patient's heart. Specifically, the radio frequency signal induces a voltage pulse in an antenna on the stimulator and that pulse is applied across a pair of electrodes, thereby stimulating adjacent muscles and contracting the heart.
Although this cardiac pacing apparatus offered several advantages over conventional pacemakers, it required that sufficient energy be derived from the received radio frequency signal to power the implanted circuit and to stimulate the adjacent organ. The amount of energy required may be relatively great, especially when the apparatus is an implantable defibrillator.
Therefore, it is desirable to provide an energy storage mechanism in the implanted apparatus which will accumulate energy from a radio frequency signal and provide that accumulated energy when needed for organ stimulation.